Backhaul downlink signal decoding method of relay station and relay station using same

ABSTRACT

A method of decoding a backhaul downlink signal of a relay node (RN). A higher layer signal indicating a maximum transmission rank is received from a base station (BS). Control information is received through a relay control channel from the BS. The control information is demodulated and mapped to resource elements (REs) which do not overlap with user equipment-specific reference signal (URS) REs in a control region which is used for the relay control channel transmission of the BS. The URS REs are reserved REs for URSs according to the maximum transmission rank. The control information is demodulated based on URSs transmitted by the BS on one fixed antenna port n, where n is a natural number.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 14/313,848 filed Jun. 24, 2014, which is a Continuation of U.S.patent application Ser. No. 13/503,740 (now U.S. Pat. No. 8,787,245,issued on Jul. 22, 2014) filed on Apr. 24, 2012, which is the NationalPhase of PCT/KR2010/007274 filed on Oct. 22, 2010, which claims priorityunder 35 U.S.C. §119(e) to U.S. Provisional Application Nos. 61/254,744filed on Oct. 25, 2009, 61/256,272 filed on Oct. 29, 2009, 61/307,409filed on Feb. 23, 2010, 61/322,816 filed on Apr. 9, 2010, 61/322,908filed on Apr. 11, 2010, 61/325,353 filed on Apr. 18, 2010, 61/334,582filed on May 14, 2010, and 61/357,513 filed on Jun. 22, 2010, and under35 U.S.C. §119(a) to Patent Application No. 10-2010-0076740 filed in theRepublic of Korea on Aug. 10, 2010, all of which are hereby expresslyincorporated by reference into the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method of decoding a backhaul downlink signalreceived by a relay station from a base station in a wirelesscommunication system including the relay station, and the relay stationusing the method.

2. Discussion of the Related Art

Standardization works of international mobile telecommunication(IMT)-advanced which is a next generation (i.e., post 3^(rd) generation)mobile communication system are carried out in the internationaltelecommunication union radio communication sector (ITU-R). TheIMT-advanced aims at support of an Internet protocol (IP)-basedmultimedia service with a data transfer rate of 1 Gbps in a stationaryor slowly moving state or 100 Mbps in a fast moving state.

3^(rd) generation partnership project (3GPP) is a system standardsatisfying requirements of the IMT-advanced, and prepares LTE-advancedwhich is an improved version of long term evolution (LTE) based onorthogonal frequency division multiple access (OFDMA)/singlecarrier-frequency division multiple access (SC-FDMA) transmission. TheLTE-advanced is one of promising candidates for the IMT-advanced. Atechnology related to a relay station is one of main technologies forthe LTE-advanced.

The relay station is a device for relaying a signal between a basestation and a user equipment, and is used for cell coverage extensionand throughput enhancement of a wireless communication system.

When the relay station receives a backhaul downlink signal from the basestation, there is an issue on which reference signal will be used todemodulate the backhaul downlink signal. For example, in order todemodulate control information of a control channel transmitted by thebase station to the relay station, there is a need to know whichreference signal is mapped to a radio resource region to which thecontrol information is allocated.

SUMMARY OF THE INVENTION

The present invention provides a method of decoding a backhaul downlinksignal received by a relay station from a base station, and alsoprovides the relay station using the method.

According to one aspect of the present invention, a method of deciding abackhaul downlink signal of a relay station is provided. The methodincludes: receiving by the relay station a transmission rank value for abackhaul downlink from a base station through a high-layer signal;receiving control information from the base station through a controlregion; and decoding the control information, wherein the transmissionrank value for the backhaul downlink is a transmission rank valueassumed when the relay station decodes the control information, andwherein the control information is mapped to a resource element whichdoes not overlap with a dedicated reference signal resource elementmapped to the control region by assuming the transmission rank value forthe backhaul downlink.

In the aforementioned aspect of the present invention, the transmissionrank value for the backhaul downlink may be equal to a maximum rankvalue that can be transmitted between the base station and at least onerelay station connected thereto.

In addition, the transmission rank value for the backhaul downlink maybe equal to a maximum rank value that can be transmitted between thebase station and the relay station.

In addition, the method further includes: receiving data from the basestation through a data region; and decoding the data. The dedicatedreference signal used to decode the data may be indicated by the controlinformation.

In addition, the control information may include a rank value for thedata region.

In addition, the high-layer signal may be a radio resource control (RRC)message.

In addition, the control region may include a plurality of orthogonalfrequency division multiplexing (OFDM) symbols in a time domain, and mayinclude OFDM symbols for transmitting a control channel by the basestation to a macro user equipment in a subframe including a plurality ofsubcarriers and at least one OFDM symbol located after a guard timerequired for transmission and reception switching of the relay stationin a frequency domain.

In addition, the method further includes: receiving control informationfrom the base station through the data region; and decoding the controlinformation received through the data region. The control informationreceived through the data region may be decoded by assuming apredetermined transmission rank value and a dedicated reference signaloverhead based on the predetermined transmission rank value.

According to another aspect of the present invention, there is provideda relay station including: a radio frequency (RF) unit for transmittingand receiving a radio signal; and a processor coupled to the RF unit,wherein the processor is configured for: receiving a transmission rankvalue for a backhaul downlink from a base station through a high-layersignal; receiving control information from the base station through acontrol region; and decoding the control information, wherein thetransmission rank value for the backhaul downlink is a transmission rankvalue assumed when the relay station decodes the control information,and wherein the control information is mapped to a resource elementwhich does not overlap with a dedicated reference signal resourceelement mapped to the control region by assuming the transmission rankvalue for the backhaul downlink.

According to the present invention, a relay station can know a referencesignal and a transmission rank used to determine an overhead of thereference signal when decoding control information received from a basestation through a high-layer signal, thereby being able to correctlydemodulate a control channel. In addition, even if a data channel and acontrol channel received from the base station have differenttransmission ranks, the control channel can be correctly demodulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system employing a relay node(RN).

FIG. 2 shows a link which exists in a wireless communication systemincluding an RN.

FIG. 3 shows a radio frame structure of 3^(rd) generation partnershipproject (3GPP) long term evolution (LTE).

FIG. 4 shows an example of a resource grid for one downlink slot.

FIG. 5 shows a structure of a downlink subframe.

FIG. 6 shows a structure of an uplink subframe.

FIG. 7 shows a multi-input multi-output (MIMO) system.

FIG. 8 shows an example of indicating a channel in a multi-antennasystem.

FIG. 9 shows an example of a reference signal (RS) structure capable ofsupporting four antenna ports in case of using a normal cyclic prefix(CP).

FIG. 10 shows an example of an RS structure capable of supporting fourantenna ports in case of using an extended CP.

FIG. 11 shows an example of a subframe structure that can be used in abackhaul downlink between an eNodeB (eNB) and an RN.

FIG. 12 shows a signaling process between an eNB and an RN when ademodulation RS (DM-RS) is used in both an R-PDCCH and an R-PDSCH.

FIG. 13 shows a relation between a DM-RS index of an R-PDCCH and a DM-RSindex of an R-PDSCH when a DM-RS set in which DM-RS indices arecontiguous is used in R-PDSCH transmission.

FIG. 14 shows an example of a reference signal resource element that canbe allocated within a backhaul downlink subframe in a normal CP.

FIG. 15 shows an example of a signaling process between an eNB and an RNwhen applying a method assuming a maximum transmission rank of abackhaul downlink.

FIG. 16 shows an example of a signaling process between an eNB and an RNwhen applying a method assuming a maximum transmission rank of abackhaul downlink in an RN-specific manner.

FIG. 17 shows an example of a DM-RS resource element assumed by an RN inan R-PDCCH region of a backhaul downlink subframe.

FIG. 18 shows an example of a DM-RS resource element of a backhauldownlink subframe.

FIG. 19 shows an exemplary structure of a transmitter according to anembodiment of the present invention.

FIG. 20 shows an example in which an eNB maps a DM-RS resource elementto an R-PDCCH region and an R-PDSCH region according to a rank.

FIG. 21 shows an example of transmitting a plurality of R-PDCCHs todifferent spatial layers when the plurality of R-PDCCHs are multiplexedin one resource block in a frequency domain.

FIG. 22 is a block diagram showing an eNB and an RN.

DETAILED DESCRIPTION OF THE INVENTION

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc.The UTRA is a part of a universal mobile telecommunication system(UMTS). 3^(rd) generation partnership project (3GPP) long term evolution(LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPPLTE uses the OFDMA in a downlink and uses the SC-FDMA in an uplink.LTE-advance (LTE-A) is an evolution of the 3GPP LTE. For clarity, thefollowing description will focus on the 3GPP LTE/LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows a wireless communication system employing a relay node(RN).

Referring to FIG. 1, the wireless communication system 10 employing theRN includes at least one eNodeB (eNB) 11. Each eNB 11 provides acommunication service to a specific geographical region 15 generallyreferred to as a cell. The cell can be divided into a plurality ofregions, and each region can be referred to as a sector. One or morecells may exist in the coverage of one eNB. The eNB 11 is generally afixed station that communicates with a user equipment (UE) 13 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, an access network (AN), etc.The eNB 11 can perform functions such as connectivity between an RN 12and a UE 14, management, control, resource allocation, etc.

The RN 12 is a device for relaying a signal between the eNB 11 and theUE 14, and is also referred to as another terminology such as a relaystation (RS), a repeater, a relay, etc. A relay scheme used in the RNmay be either amplify and forward (AF) or decode and forward (DF), andthe technical features of the present invention are not limited thereto.

The UEs 13 and 14 may be fixed or mobile, and may be referred to asanother terminology, such as a mobile station (MS), a user terminal(UT), a subscriber station (SS), a wireless device, a personal digitalassistant (PDA), a wireless modem, a handheld device, an access terminal(AT), etc. Hereinafter, a macro UE (or Ma-UE) 13 denotes a UE thatdirectly communicates with the eNB 11, and a relay node-UE (RN-UE) 14denotes a UE that communicates with the RN. Even if the Ma-UE 13 existsin a cell of the eNB 11, the Ma-UE 13 can communicate with the eNB 11via the RN 12 to improve a data transfer rate depending on a diversityeffect.

FIG. 2 shows a link which exists in a wireless communication systemincluding an RN.

The wireless communication system including the RN located between aneNB and a UE may have a link different from that of a wirelesscommunication system having only the eNB and the UE. Between the eNB andthe UE, a downlink implies a communication link from the eNB to the UE,and an uplink implies a communication link from the UE to the eNB. Whenusing time division duplex (TDD), downlink transmission and uplinktransmission are performed in different subframes. When using frequencydivision duplex (FDD), downlink transmission and uplink transmission areperformed in different frequency bands. In the TDD, downlinktransmission and uplink transmission are performed at different timesand can use the same frequency band. On the other hand, in the FDD,downlink transmission and uplink transmission can be performed at thesame time, and use different frequency bands.

When the RN is located between the eNB and the UE, a backhaul link andan access link can be added in addition to the aforementioned uplink anddownlink. The backhaul link refers to a communication link between theeNB and the RN, and includes a backhaul downlink on which the eNBtransmits a signal to the RN and a backhaul uplink on which the RNtransmits a signal to the eNB. The access link refers to a communicationlink between the RN and the UE connected to the RN (hereinafter, such aUE is referred to as an RN-UE). The access link includes an accessdownlink on which the RN transmits a signal to the RN-UE and an accessuplink on which the RN-UE transmits a signal to the RN.

FIG. 3 shows a radio frame structure of 3GPP LTE.

Referring to FIG. 3, a radio frame consists of 10 subframes. Onesubframe consists of 2 slots. One subframe may have a length of 1millisecond (ms), and one slot may have a length of 0.5 ms. A time fortransmitting one subframe is defined as a transmission time interval(TTI). The TTI may be a minimum unit of scheduling.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesOFDMA in downlink transmission, the OFDM symbol is for representing onesymbol period, and can be referred to as other terms. For example, theOFDM symbol can also be referred to as an SC-FDMA symbol when SC-FDMA isused as an uplink multiple-access scheme. Although it is describedherein that one slot includes 7 OFDM symbols, the number of OFDM symbolsincluded in one slot may change depending on a cyclic prefix (CP)length. According to 3GPP TS 36.211 V8.5.0(2008-12), in case of a normalCP, one subframe includes 7 OFDM symbols, and in case of an extended CP,one subframe includes 6 OFDM symbols. The radio frame structure is forexemplary purposes only, and thus the number of subframes included inthe radio frame and the number of slots included in the subframe maychange variously. Hereinafter, a symbol may imply one OFDM symbol or oneSC-FDMA symbol.

The sections 4.1 and 4.2 of 3GPP TS 36.211 V8.3.0 (2008-05) “TechnicalSpecification Group Radio Access Network; Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modulation (Release 8)” canbe incorporated herein by reference to explain the radio frame structuredescribed with reference to FIG. 3.

FIG. 4 shows an example of a resource grid for one downlink slot.

In FDD and TDD radio frames, one slot includes a plurality of OFDMsymbols in a time domain and a plurality of resource blocks (RBs) in afrequency domain. The RB is a resource allocation unit, and includes aplurality of consecutive subcarriers in one slot.

Referring to FIG. 4, although it is described herein that one downlinkslot includes 7 OFDM symbols and one RB includes 12 subcarriers in thefrequency domain, this is for exemplary purposes only, and thus thepresent invention is not limited thereto. A subcarrier spacing may be,for example, 15 kHz in the RB.

Each element on the resource grid is referred to as a resource element,and one RB includes 12×7 resource elements. The number N^(DL) of RBsincluded in the downlink slot depends on a downlink transmissionbandwidth determined in a cell. The resource grid described in FIG. 4can also apply to uplink transmission.

FIG. 5 shows a structure of a downlink subframe.

Referring to FIG. 5, the subframe includes two consecutive slots. Amaximum of three OFDM symbols located in a front portion of a 1^(st)slot within the subframe correspond to a control region to which aphysical downlink control channel (PDCCH) is allocated. The remainingOFDM symbols correspond to a data region to which a physical downlinkshared channel (PDSCH) is allocated. In addition to the PDCCH, controlchannels such as a physical control format indicator channel (PCFICH), aphysical hybrid automatic repeat request (HARM) indicator channel(PHICH), etc., can be allocated to the control region. A UE can readdata information transmitted through the PDSCH by decoding controlinformation transmitted through the PDCCH. Although the control regionincludes three OFDM symbols herein, this is for exemplary purposes only.Thus, two OFDM symbols or one OFDM symbol may be included in the controlregion. The number of OFDM symbols included in the control region of thesubframe can be known by using the PCFICH. The PHICH carries informationindicating whether uplink data transmitted by the UE is successfullyreceived.

The control region consists of a plurality of control channel elements(CCEs) as a logical CCE stream. Hereinafter, the CCE stream denotes aset of all CCEs constituting the control region in one subframe. The CCEcorresponds to a plurality of resource element groups (REGs). Forexample, the CCE may correspond to 9 REGs. The REG is used to definemapping of a control channel onto a resource element. For example, oneREG may consist of four resource elements.

A plurality of PDCCHs may be transmitted in the control region. ThePDCCH carries control information such as scheduling allocation. ThePDCCH is transmitted on an aggregation of one or several consecutiveCCEs. A PDCCH format and the number of available PDCCH bits aredetermined according to the number of CCEs constituting the CCEaggregation. The number of CCEs used for PDCCH transmission is referredto as a CCE aggregation level. In addition, the CCE aggregation level isa CCE unit for searching for the PDCCH. A size of the CCE aggregationlevel is defined by the number of contiguous CCEs. For example, the CCEaggregation level may be an element of {1, 2, 4, 8}.

Control information transmitted through the PDCCH is referred to asdownlink control information (hereinafter, DCI). The DCI includes uplinkscheduling information, downlink scheduling information, systeminformation, an uplink power control command, control information forpaging, control information for indicating a random access channel(RACH) response, etc.

Examples of a DCI format include a format 0 for scheduling of a physicaluplink shared channel (PUSCH), a format 1 for scheduling of one physicaldownlink shared channel (PDSCH) codeword, a format 1A for compactscheduling of the one PDSCH codeword, a format 1B for simple schedulingfor rank-1 transmission of a single codeword in a spatial multiplexingmode, a format 1C for significantly compact scheduling of a downlinkshared channel (DL-SCH), a format 1D for scheduling of the PDSCH in amulti-user spatial multiplexing mode, a format 2 for scheduling of thePDSCH in a closed-loop spatial multiplexing mode, a format 2A forscheduling of the PDSCH in an open-loop spatial multiplexing mode, aformat 3 for transmission of a transmission power control (TPC) commandfor 2-bit power control for the PUCCH and the PUSCH, and a format 3A fortransmission of a TPC command for 1-bit power control for the PUCCH andthe PUSCH.

FIG. 6 shows a structure of an uplink subframe.

Referring to FIG. 6, the uplink subframe can be divided into a controlregion and a data region in frequency domain. The control region is aregion to which a physical uplink control channel (PUCCH) for carryinguplink control information is allocated. The data region is a region towhich a physical uplink shared channel (PUSCH) for carrying user data isallocated.

The PUCCH for one UE is allocated in a pair of RBs. The RBs belonging tothe RB pair occupy different subcarriers in each of two slots. This iscalled that the RB pair allocated to the PUCCH is frequency-hopped in aslot boundary.

The PUCCH can support multiple formats. That is, uplink controlinformation having a different number of bits for each subframe can betransmitted according to a modulation scheme. For example, when using abinary phase shift keying (BPSK) (i.e., a PUCCH format 1a), 1-bit uplinkcontrol information can be transmitted on the PUCCH, and when usingquadrature phase shift keying (QPSK) (i.e., a PUCCH format 1b), 2-bituplink control information can be transmitted on the PUCCH. In additionthereto, examples of the PUCCH format include a format 1, a format 2, aformat 2a, a format 2b, etc. For this, the section 5.4 of 3GPP TS 36.211V8.2.0 (2008-03) “Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channelsand Modulation (Release 8)” can be incorporated herein by reference.

A wireless communication system, for example, the wireless communicationsystem described with reference to FIG. 1, may be a system using amulti-input multi-output (MIMO) technique, that is, may be a MIMOsystem. Being evolved from the conventional technique in which a singletransmit (Tx) antenna and a single receive (Rx) antenna are used, theMIMO technique uses multiple Tx antennas and multiple Rx antennas toimprove transfer efficiency of data to be transmitted or received. Inother words, the MIMO technique is a technique of using a plurality ofantennas in a transmitter or a receiver of the wireless communicationsystem. Performance and communication capacity of the wirelesscommunication system can be improved by using the MIMO technique. TheMIMO system is also referred to as a multi-antenna system. In the MIMOtechnique, instead of receiving one whole message through a singleantenna path, data segments are received through a plurality of antennasand are then collected as one piece of data. As a result, a datatransfer rate can be improved in a specific range, or a system range canbe increased with respect to a specific data transfer rate.

A next-generation mobile communication technique requires a datatransfer rate higher than that used in the conventional mobilecommunication technique. Therefore, a MIMO technique is essential to thenext-generation mobile communication technique. The MIMO technique canbe applied not only to an eNB but also to a UE or an RN, and thus can beused to overcome a limitation of a data transfer rate. In addition, theMIMO technique is drawing attention more than various other techniquesdue to a technical advantage in that data transmission efficiency can beimproved without having to use an additional frequency band or withouthaving to require additional transmission power.

First, mathematical modeling of a MIMO system will be described.

FIG. 7 shows a MIMO system.

Referring to FIG. 7, a transmitter 700 has N_(T) Tx antennas, and areceiver 800 has N_(R) Rx antennas. In this case, ideal channeltransmission capacity is increased in proportion to the number ofantennas.

In theory, a data transfer rate obtained by the increase in channeltransmission capacity can be expressed by the product between a maximumdata rate R_(O) obtained when using a single antenna and an incrementrate R_(i) generated when using multiple antennas. The increment rateR_(i) can be expressed by Equation 1 below.R _(i)=min(N _(T) ,N _(R)  [Equation 1]

If N_(T) denotes the number of Tx antennas, transmission information mayconsist of up to N_(T) different pieces of information. In this case,the transmission information can be expressed by Equation 2 below.s=[s ₁ , s ₂ , . . . , s _(N) _(T) ]^(T)  [Equation 2]

In Equation 2, s denotes a transmission information vector, and s1, s2,. . . , sNT denote information indicating each element of thetransmission information vector. Each information can be transmittedwith different transmission power. When each transmission power isdenoted by (P1, P2, . . . , PNT), the transmission information vector towhich the transmission power is allocated can be expressed by Equation 3below.ŝ=[ŝ ₁ , ŝ ₂ , . . . , ŝ _(N) _(T) ]^(T) =[P ₁ S ₁ , P ₂ s ₂ , . . . , P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

Equation 3 can be expressed by the product between a transmission powerdiagonal matrix and a transmission information vector as shown inEquation 4 below.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

A transmission information vector to which transmission power is appliedis multiplied by a weight matrix W to generate Tx signals x₁, x₂, . . ., x_(NT) transmitted in practice through N_(T) Tx antennas. The weightmatrix W takes a role of properly distributing transmission informationto an individual antenna according to a transmission channel condition.If a Tx signal vector is denoted by x, it can be expressed by Equation 5below.

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix}{\quad{= {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{{i\; 1}\;} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In Equation 5, elements w_(ij) (1≦i≦N_(T), 1≦j≦N_(T)) of the weightmatrix denote a weight for an i^(th) Tx antenna and j^(th) transmissioninformation. The weight matrix W is also referred to as a precodingmatrix.

The Tx signal vector can include different transmission informationaccording to a transmission scheme. For example, when applying spatialdiversity, i.e., transmission diversity, all elements of the Tx signalvector may have the same transmission information. That is, [s_(i), s₂,. . . , s_(nT)] may be the same information, for example, [s_(i), s₁, .. . , s₁]. Therefore, since the same transmission information isdelivered to a receiver through a different channel, a diversity effectoccurs, and transmission reliability increases.

Alternatively, when applying the spatial multiplexing, all elements ofthe transmission information of the Tx signal vector may be differentfrom one another. That is, s₁, s₂, . . . s_(nT) may be differentinformation. Since different transmission information is delivered tothe receiver through a different channel, advantageously, there is anincrease in an amount of information that can be transmitted.

Of course, the transmission information can be delivered by usingspatial multiplexing together with the spatial diversity. That is, inthe above example, the same information is transmitted by using thespatial diversity through three Tx antennas, and different informationcan be transmitted by using the spatial multiplexing through theremaining Tx antennas. In this case, the transmission information vectorcan be configured such as [s₁, s₁, s₁, s₂, s₃ . . . s_(nT-2)].

If N_(R) denotes the number of Rx antennas in the receiver, a signalreceived in an individual Rx antenna can be denoted by y_(n)(1≦n≦N_(R)).In this case, an Rx signal vector y can be expressed by Equation 6below.y=[y ₁ , y ₂ , . . . , y _(N) _(R) ]^(T)  [Equation 6]

When performing channel modeling in the MIMO system, each channel can beidentified by using an index of a Tx antenna and an index of an Rxantenna. If the index of the Tx antenna is denoted by j and the index ofthe Rx antenna is denoted by i, a channel between the Tx antenna and theRx antenna can be denoted by h_(ij) (herein, it should be noted that theindex of the Rx antenna is first indicated in a subscript indicating thechannel and the index of the Tx antenna is indicated later).

FIG. 8 shows an example of indicating a channel in a multi-antennasystem.

Referring to FIG. 8, channels for respective N_(T) Tx antennas withrespect to an Rx antenna i are denoted by h_(i1), h_(i2), . . . ,h_(iNT). For convenience of explanation, the channels can be expressedas a matrix or a vector. Then, the channels h_(i1), h_(i2), . . . ,h_(iNT) can be expressed in a vector form as shown in Equation 7 below.h _(i) ^(T) =[h _(i1) , h _(i2) , . . . , h _(iN) _(T) ]  [Equation 7]

If all channels from N_(T) Tx antennas to N_(R) Rx antennas areexpressed in a matrix form as a channel matrix H, the channel matrix Hcan be expressed by Equation 8 below.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{{i\; 1}\;} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

A signal transmitted through a Tx antenna is transmitted through achannel expressed by Equation 8 above and is received in an Rx antenna.In this case, noise is added in an actual channel. Mathematically, thenoise can be regarded as an additive white Gaussian noise (AWGN). IfAWGNs added to respective Rx antennas are denoted by n₁, n₂, . . . ,n_(NR), for convenience of explanation, these AWGNs can be expressed asa vector of Equation 9 below.n=[n ₁ , n ₂ , . . . , n _(N) _(R) ]^(T)  [Equation 9]

An Rx signal vector y received in an Rx antenna can be expressed byEquation 10 below by considering the aforementioned AWGN, the Tx signalvector x, a channel matrix, etc.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{{i\; 1}\;} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In the channel matrix H, the number of rows and the number of columnsare determined according to the number of Tx antennas and the number ofRx antennas. In the channel matrix H, the number of rows is equal to thenumber of Rx antennas. Further, in the channel matrix H, the number ofcolumns is equal to the number of Tx antennas. Therefore, the channelmatrix H can be expressed by an N_(R)×N_(T) matrix.

In general, a matrix rank is defined by a smaller value between thenumber of independent rows and the number of independent columns.Therefore, the matrix rank cannot be greater than the number of columnsor the number of rows, and a rank of the channel matrix H is determinedby Equation 11 below.rank(H)≦min(N _(T) ,N _(R))  [Equation 11]

In general, transmission information (e.g., data) is easily distortedand modified while it is transmitted through a radio channel. Therefore,a reference signal (RS) is necessary to demodulate the transmissioninformation in an error-free manner. The RS is a signal pre-knownbetween the transmitter and the receiver and is transmitted togetherwith the transmission information. Since the transmission informationtransmitted from the transmitter experiences a corresponding channel foreach Tx antenna or for each layer, the RS can be allocated for each Txantenna or for each layer. The RS for each Tx antenna or for each layercan be identified by using a resource (e.g., time, frequency, code,etc.). The RS can be used for two purposes, i.e., transmissioninformation demodulation and channel estimation.

The RS can be classified into two types according to a range of areceiver which knows the RS in advance. A first type of the RS is knownto only a specific receiver (e.g., a specific UE), and is called adedicated RS (DRS). In this sense, the DRS is also called a UE-specificRS. A second type of the RS is known to all receivers in a cell, e.g.,all UEs, and is called a common RS (CRS). The CRS is also called acell-specific RS.

In addition, the RS can be classified according to a usage. For example,an RS used for data demodulation is called a demodulation RS (DM-RS). AnRS used for feedback information indicating a channel state (e.g.,CQI/PMI/RI) is called a channel state indicator-RS (CSI-RS). Theaforementioned DRS can be used as a DM-RS. Hereinafter, it is premisedthat the DM-RS is a DRS.

FIG. 9 shows an example of an RS structure capable of supporting fourantenna ports in case of using a normal CP. FIG. 10 shows an example ofan RS structure capable of supporting four antenna ports in case ofusing an extended CP. The RS structures of FIG. 9 and FIG. 10 are RSstructures used in the conventional 3GPP LTE system.

In FIG. 9 and FIG. 10, a resource element indicated by any one of values0 to 3 indicates a resource element for transmitting a cell-specific RS(CRS). In this case, any one value in the range of 0 to 3 indicates asupported antenna port. That is, resource elements marked with p (wherep is any one of values 0 to 3) are resource elements to which a CRS foran antenna port p is mapped. The CRS is used for channel measurement anddata demodulation for each antenna port. The CRS is transmitted in botha control region and a data region of a subframe.

In FIG. 9 and FIG. 10, a resource element marked with ‘D’ indicates aresource element to which a UE-specific RS (i.e., a DRS) is mapped. TheUE-specific RS can be used in single-antenna port transmission of aPDSCH. A UE receives an indication on whether the UE-specific RS istransmitted through a high-layer signal, and if the PDSCH istransmitted, whether the UE-specific RS is valid. The UE-specific RS canbe transmitted only when data demodulation is necessary. The UE-specificRS can be transmitted only in a data region of a subframe.

Now, a subframe structure that can be applied to a backhaul downlinkbetween an eNB and an RN will be described, and then a reference signalthat can be used in the backhaul downlink will be described.

First, terminologies will be described for convenience of explanation.Hereinafter, an R-PDCCH is a physical control channel for transmittingcontrol information by the eNB to the RN, and an R-PDSCH is a physicaldata channel for transmitting data by the eNB to the RN. Hereinafter, an‘x’ region is a radio resource region for transmitting ‘x’. For example,an R-PDCCH region is a radio resource region for transmitting theR-PDCCH by the eNB.

FIG. 11 shows an example of a subframe structure that can be used in abackhaul downlink between an eNB and an RN.

Referring to FIG. 11, the eNB transmits a PDCCH (also can be referred toas a macro PDCCH) to a macro UE (Ma-UE) in a specific number of firstOFDM symbols of a subframe. In the specific number of first OFDMsymbols, the RN can transmit the PDCCH to an RN-UE. The RN cannotreceive a backhaul signal from the eNB due to self-interference in anOFDM symbol duration in which the PDCCH is transmitted to the RN-UE.

The eNB transmits a backhaul signal to the RN after a guard time (GT)elapses. The GT is a stabilization period depending on signaltransmission/reception switching of the RN. In FIG. 11, a case where theGT corresponds to one OFDM symbol is exemplified. However, the GT may beequal to or less than one OFDM symbol duration, and optionally may beequal to or greater than one OFDM symbol. In addition, the GT can be setto a duration of an OFDM symbol unit in a time domain, and can be set toa sampling time unit. Although the GT is located in both front and rearparts of a backhaul reception duration in FIG. 11, the present inventionis not limited thereto. That is, the GT located in the rear part of thebackhaul reception duration in the time domain may not be set accordingto a timing alignment relation of the subframe. In this case, thebackhaul reception duration can be extended up to a last OFDM symbol ofthe subframe. The GT can be defined only for a frequency band configuredto transmit a signal by the eNB to the RN.

The eNB can allocate the backhaul downlink resource to be allocated tothe RN by classifying the backhaul downlink resource into two types.

One type is a primary backhaul region and is a resource region in whichan R-PDCCH and an R-PDSCH can be transmitted. In the primary backhaulregion, the R-PDCCH and the R-PDSCH can be multiplexed using timedivision multiplexing (TDM). That is, the R-PDCCH and the R-PDSCH can betransmitted by being divided in a time domain, and the R-PDSCH can belocated after the R-PDCCH. The R-PDCCH included in the primary backhaulregion can include resource allocation information regarding not onlythe R-PDSCH of a frequency band at which the R-PDCCH is transmitted butalso regarding the R-PDSCH located at another frequency band. Inaddition, although it is shown in FIG. 11 that the R-PDSCH is alsotransmitted in the primary backhaul region, the present invention is notlimited thereto. That is, only the R-PDCCH can be transmitted withouthaving to transmit the R-PDSCH in all OFDM symbols of the primarybackhaul region.

The other type is a secondary backhaul region. Only the R-PDSCH istransmitted in the secondary backhaul region, and can be indicated bythe R-PDCCH included in the primary backhaul region as described above.

The backhaul signal transmitted in the primary backhaul region and thesecondary backhaul region can be transmitted by being multiplexed with aPDSCH transmitted to a Ma-UE in a frequency domain.

Which reference signal will be used in the R-PDCCH and the R-PDSCH ofthe backhaul downlink subframe needs to be taken into consideration.

The present invention proposes to use a DM-RS (DRS) for bothtransmission (from the perspective of the eNB)/reception (from theperspective of the RN) of the R-PDCCH and the R-PDSCH. This method isadvantageous when applying improved multi-user (MU) MIMO (e.g.,zero-forcing MU-MIMO) to a region in which a backhaul signal istransmitted. In other words, since the DM-RS is applied to the entirebackhaul signal (including both the R-PDCCH and the R-PDSCH) transmittedby the eNB, the R-PDCCH and the R-PDSCH can be spatially multiplexedwith another backhaul signal, and can also be spatially multiplexed withthe PDSCH transmitted to a Ma-UE.

Hereinafter, a method of signaling between an eNB and an RN and a methodof operating the RN will be described when the DM-RS is used in both anR-PDCCH and an R-PDSCH.

1. Signaling Between eNB and RN

FIG. 12 shows a signaling process between an eNB and an RN when a DM-RSis used in both an R-PDCCH and an R-PDSCH.

Referring to FIG. 12, the eNB can report an index of the DM-RS used inthe R-PDCCH through a high-layer signal (e.g., a radio resource control(RRC) message) (step S100). Herein, the index of the DM-RS collectivelyrefers to information capable of identifying the DM-RS. Examples of theindex of the DM-RS include information on an antenna port fortransmitting the DM-RS of the R-PDCCH for each RN, information on ascramble identifier (ID) applied to an antenna port 0 for transmittingthe DM-RS of the R-PDCCH for each RN, or a combination of theaforementioned scramble ID and the antenna port for transmitting theDM-RS of the R-PDCCH. The scramble ID of the antenna port fortransmitting the DM-RS must be different from the scramble ID of theDM-RS antenna port that can be used for scheduling of differentmulti-user MIMO resources in a space domain.

A UE performs macro PDCCH decoding by using a CRS, and thus can know anindex of a DM-RS used for decoding of a macro PDSCH. However, an RN maynot be able to decode the macro PDCCH transmitted by an eNB. This isbecause the RN may transmit the PDCCH to an RN-UE during the eNBtransmits the macro PDCCH. That is, since the RN cannot receive themacro PDCCH from the eNB during the PDCCH is transmitted to the RN-UE,the RN cannot decode the macro PDCCH. Therefore, the eNB must report theindex of the DM-RS used in the R-PDCCH to the RN through the high-layersignal.

By considering a fact that the R-PDCCH must be transmitted with highreliability even though it includes a limited number of bits, atransmission rank of the R-PDCCH can be limited to a specific value. Forexample, the transmission rank of the R-PDCCH can be limited to 1. Thatis, the eNB may not use spatial multiplexing in the R-PDCCH transmittedto the RN.

Alternatively, the eNB may use spatial multiplexing in R-PDCCHtransmission. The eNB can transmit a transmission rank value of theR-PDCCH to the RN through the high-layer signal (e.g., an RRC message)in order to avoid a situation where the RN has to perform blind decodingor has to perform blind detection on the transmission rank of theR-PDCCH. When the transmission rank value of the R-PDCCH is given, theRN can recognize a location and the total number of resource elements towhich a DM-RS is allocated in an R-PDCCH region.

Although FIG. 12 shows an example in which the eNB transmits the indexof the DM-RS for the R-PDCCH and/or the transmission rank value of theR-PDCCH through the high-layer signal, in order to decrease signalingoverhead, the DM-RS index or the transmission rank value of the R-PDCCHcan be pre-set to a specific value.

The eNB can report the index of the DM-RS used in the R-PDSCH throughcontrol information included in the R-PDCCH (step S200). In this case,an amount of the control information included in the R-PDCCH can bedecreased by determining a specific relation between a DM-RS set used inthe R-PDCCH and a DM-RS set used in the R-PDSCH.

For example, regarding a primary backhaul region, the DM-RS used in theR-PDCCH can be equally used for the R-PDSCH. In other words, regardingthe R-PDCCH and R-PDSCH included in the primary backhaul region, it canbe determined such that the DM-RS used in the R-PDCCH is always used inthe R-PDSCH. That is, a DM-RS set used in the R-PDCCH can be expressedas a subset of the DM-RS set used in the R-PDSCH.

A relation determined as described above can equally apply to asecondary backhaul region. That is, the DM-RS set used in the R-PDCCHincluded in the primary backhaul region is a subset of the DM-RS setused in the R-PDSCH included in the secondary backhaul region. In otherwords, the DM-RS used in the R-PDCCH included in the primary backhaulregion is always used in the R-PDSCH included in the secondary backhaulregion.

When the DM-RS set used in the R-PDSCH is determined as described above,a control information signaling overhead for reporting the DM-RS setused in the R-PDSCH can be decreased. This is because one DM-RS indexused in the R-PDSCH (i.e., the DM-RS index used in the R-PDCCH) is knownto the eNB and the RN through a high-layer signal, and thus the DM-RSindex can be omitted from the control information of the R-PDCCH.

In addition, the DM-RS provides a beamforming gain in comparison with aCRS/CSI-RS. For example, when a bitmap is used to indicate the DM-RSindex for the R-PDSCH, the DM-RS index used in the R-PDCCH can beexcluded from the bitmap. This is because it is known to the RN that theDM-RS used in the R-PDCCH is used as the DM-RS of the R-PDSCH asdescribed above.

For another example, if a DM-RS set in which DM-RS indices arecontiguous is used in R-PDSCH transmission, it is enough to report atransmission rank value of the R-PDSCH through the R-PDCCH. That is, ifn denotes an index of the DM-RS used in the R-PDCCH, DM-RS indices usedfor the R-PDSCH may be n, n+1, . . . , n+k−1, where k denotes thetransmission rank value of the R-PDSCH.

FIG. 13 shows a relation between a DM-RS index of an R-PDCCH and a DM-RSindex of an R-PDSCH when a DM-RS set in which DM-RS indices arecontiguous is used in R-PDSCH transmission.

If the DM-RS index value n of the DM-RS used in the R-PDCCH is reportedthrough a high-layer signal and a transmission rank value k of theR-PDSCH is reported through control information of the R-PDCCH, theDM-RS of the R-PDSCH can have values of the DM-RS indices n, n+1, . . ., n+k−1.

Referring back to FIG. 12, the RN decodes the R-PDCCH (step S300). Bydecoding the R-PDCCH, the RN can know a correct set of DM-RS used in theR-PDSCH. Further, the eNB transmits the R-PDSCH (step S400), and the RNreceives and decodes the R-PDSCH (step S500). Although it is shown inFIG. 12 that the RN decodes the R-PDCCH and then the eNB transmits theR-PDSCH, this is for exemplary purposes only, and thus the presentinvention is not limited thereto. That is, the RN may receive both theR-PDCCH and the R-PDSCH and then decode the R-PDCCH and the R-PDSCH inthat order, or R-PDCCH decoding and R-PDSCH reception may be performedsimultaneously.

2. Resource Element Mapping Used in R-PDCCH and R-PDSCH

Hereinafter, a method in which an eNB determines a resource element usedin an R-PDCCH and an R-PDSCH will be described.

FIG. 14 shows an example of a reference signal resource element that canbe allocated within a backhaul downlink subframe in a normal CP.

Referring to FIG. 14, the reference signal resource element is allocatedin a specific pattern to a region including one subframe in a timedomain and including 12 subcarriers in a frequency domain (forconvenience of explanation, such a region is called a basic unitregion). For example, in each slot, a reference signal resource elementfor a CRS can be allocated with a spacing of three subcarriers to1^(st), 2^(nd), 5^(th) OFDM symbols (if OFDM symbols in a slot areindexed sequentially from 0, it can be expressed by an OFDM symbol #0,an OFDM symbol #1, and an OFDM symbol #4). A reference signal resourceelement for a DM-RS (DRS) (hereinafter, a DM-RS resource element) can beallocated to 6^(th) and 7^(th) OFDM symbols in each slot.

In case of the DM-RS (DRS), for transmission of up to a rank 2, 12resource elements are used in the basic unit region, and fortransmission of a rank 3 or a higher rank, 12 resource elements areadditional used in the basis unit region in addition to the 12 resourceelements for transmission of up to the rank 2, and thus 24 resourceelements are used in total (of course, the number of resource elementsused depending on the rank is for exemplary purposes only, and thus thenumber of resource elements may vary). That is, the number of the DM-RSresource elements and a pattern thereof are determined according to atransmission rank of an R-PDSCH.

A conventional RN can know the transmission rank of the R-PDSCH onlyafter decoding an R-PDCCH. That is, the RN cannot know the transmissionrank of the R-PDSCH before decoding the R-PDCCH. However, there is aproblem in that the RN uses the DM-RS to decode the R-PDCCH, and theDM-RS resource element can vary depending on the transmission rank ofthe R-PDSCH.

For example, when four OFDM symbols (from a 4^(th) OFDM symbol to a7^(th) OFDM symbol of a 1^(st) slot) are used as the R-PDCCH as shown inFIG. 14, the RN cannot know whether the number of DM-RS resourceelements in the basic unit region is 12 or 24, and cannot know the DM-RSresource element included in the four OFDM symbols. Therefore, the RNhas to decode the R-PDCCH through blind decoding. That is, the R-PDCCHis decoded by using a method of performing decoding on all possibleresource element combinations in the four OFDM symbols. As a result, anoverhead of a receiver of the RN is excessively increased.

In order to solve such a problem, the eNB can restrict an R-PDCCHresource element (i.e., a resource element to which control informationof the R-PDCCH is mapped) to a resource element which does not overlapwith all resources that can be used in DM-RS transmission (such aresource element is called a DM-RS candidate resource element). That is,the eNB can perform transmission by puncturing all DM-RS candidateresource elements to which the DM-RS can be allocated in the R-PDCCHregion and by mapping control information transmitted through theR-PDCCH to the remaining resource elements. Herein, a resource elementof all candidate locations to which a CSI-RS can be additionallyallocated can also be excluded. In this case, the eNB can report theCSI-RS through system information, and the RN can know in advance abouta specific resource element through which the CSI-RS is transmitted. TheRN can decode the R-PDCCH under the assumption that the DM-RS resourceelement has a pattern of the DM-RS candidate resource element, i.e., apattern depending a maximum transmission rank value of the R-PDSCH.

Any one of two methods described below can be used to implement theaforementioned method in which the eNB restricts the R-PDCCH resourceelement to a resource element which does not overlap with the DM-RScandidate resource element and the RN decodes the R-PDCCH and theR-PDSCH.

1. Method Assuming Maximum Transmission Rank of all Backhaul Links

For example, assume that an eNB communicates with an RN 1 and an RN 2.In this case, let assume that a maximum transmission rank is 2 in abackhaul downlink between the eNB and the RN 1, and the maximumtransmission rank is 8 in a backhaul downlink between the eNB and the RN2. Then, a maximum transmission rank of all backhaul downlinks is 8.

In this case, by assuming that the maximum transmission rank of thebackhaul downlink is 8 for both the RN 1 and the RN 2, the eNB can mapR-PDCCH resource elements in the R-PDCCH region. And, when the RN 1 andthe RN 2 both decode their R-PDCCHs, it is assumed that the DM-RSresource elements are mapped for a case of a rank 3 or a higher rank andit is assumed that the remaining resource elements other than the DM-RSresource elements in the R-PDCCH region are the R-PDCCH resourceelements. In other words, both the RN 1 and the RN 2 decode the R-PDCCHby using resource elements which do not overlap with the DM-RS resourceelements in the R-PDCCH region by assuming that a DM-RS is mapped to 24resource elements in a basic unit region.

For another example, when the eNB communicates with the RN 1 and the RN2, a maximum transmission rank in a backhaul downlink between the eNBand the RN 1 may be 2, and a maximum transmission rank in a backhauldownlink between the eNB and the RN 2 may also be 2. Herein, the maximumtransmission rank of the all backhaul downlinks is 2. In this case,there is no possibility that transmission with a rank 3 or a higher rankis performed in a backhaul downlink between the eNB and all RNs.Therefore, the eNB maps R-PDCCH resource elements to an R-PDCCH regionfor a case where 12 DM-RS resource elements exist in a basic unitregion, and each RN can decode the R-PDCCH by assuming an R-PDCCH regionfor a case where 12 DM-RS resource elements exist in the basis unitregion.

FIG. 15 shows an example of a signaling process between an eNB and an RNwhen applying the aforementioned method assuming the maximumtransmission rank of the backhaul downlink.

The eNB transmits a maximum transmission rank value of all backhauldownlink through a high-layer signal such as an RRC message (step S101).The maximum transmission rank value of the all backhaul downlink isequal to the maximum number of independent streams that can betransmitted in the backhaul downlink. Although not shown in the figure,the eNB can also transmit an index of a DM-RS for an R-PDCCH through thehigh-layer signal. The maximum transmission rank value for the allbackhaul downlink is a transmission rank value used to determine anoverhead of a DM-RS for a case of decoding control information receivedby the RN from the eNB, that is, control information received throughthe R-PDCCH, and is a value which is assumed by the eNB and the RN.

The eNB transmits to the RN an R-PDCCH to which the DM-RS for which themaximum transmission rank value of the all backhaul downlink is assumed(step S201). The RN decodes R-PDCCH control information by assumingDM-RS mapping for a case where the R-PDSCH region has the maximumtransmission rank value of the all backhaul downlink (step S301). TheeNB transmits the R-PDSCH to the RN (step S401). The RN decodes theR-PDSCH (step S501). As described above, the R-PDSCH is decoded by usingthe DM-RS mapped according to an actual transmission rank value.

Although FIG. 15 shows an example in which a data channel for the RN,i.e., an R-PDSCH, is transmitted in the R-PDSCH region, a controlchannel may be optionally transmitted in the R-PDSCH region. In thiscase, for convenience of explanation, a control channel transmitted foran RN in the R-PDCCH region can be called a 1^(st) R-PDCCH, and acontrol channel transmitted for an RN in the R-PDSCH region can becalled a 2^(nd) R-PDCCH. The 1^(st) R-PDCCH and the 2^(nd) R-PDCCH caninclude control information for the same RN, and can include informationon different RNs. In this case, an actual transmission rank of the2^(nd) R-PDCCH and an overhead of the DM-RS depending on thetransmission rank (i.e., a pattern or the number of DM-RSs in a basicunit region) can be assumed to be a pre-defined value (e.g., atransmission rank 1 or an overhead of a DM-RS depending thereon).

2. Method Assuming Maximum Transmission Rank of Backhaul Downlink inRN-Specific Manner

An eNB can map R-PDCCH resource elements by assuming a maximumtransmission rank for an individual backhaul downlink of each RNreported to each RN through a high-layer signal.

For example, it is assumed that a maximum transmission rank of abackhaul downlink between the eNB and the RN 1 is 2, and a maximumtransmission rank of a backhaul downlink between the eNB and the RN 2 is8.

In this case, by assuming that the maximum transmission rank of the RN 1is 2, the eNB can map DM-RS resource elements in an R-PDCCH region andcan map R-PDCCH control information to resource elements which do notoverlap with the DM-RS resource elements. Then, by assuming a case wherea DM-RS is mapped to 12 resource elements in a basic unit region, the RN1 decodes an R-PDCCH by using resource elements which do not overlapwith the DM-RS resource elements in the R-PDCCH region.

Under the assumption that the maximum transmission rank of the RN 2 is8, the eNB can map the DM-RS resource elements to the R-PUCCH region andcan map the R-PDCCH control information to the resource elements whichdo not overlap with the DM-RS resource elements. Then, by assuming acase where a DM-RS is mapped to 24 resource elements in a basic unitregion, the RN 2 decodes an R-PDCCH by using resource elements which donot overlap with the DM-RS resource elements in the R-PDCCH region.

FIG. 16 shows an example of a signaling process between an eNB and an RNwhen applying the aforementioned method assuming the maximumtransmission rank of the backhaul downlink in the RN-specific manner.

The eNB transmits a maximum transmission rank value of an RN-specificbackhaul downlink through a high-layer signal such as an RRC message(step S102). The maximum transmission rank value of the RN-specificbackhaul downlink is a transmission rank value used to determine anoverhead of a DM-RS in a case of decoding control information receivedby each RN from the eNB, i.e., control information received through theR-PDCCH, and is a value which is assumed by the eNB and the RN. Themaximum transmission rank value of the RN-specific backhaul downlink canvary depending on each RN. Although not shown in the figure, the eNB canalso transmit an index of a DM-RS for an R-PDCCH through a high-layersignal.

The eNB transmits to the RN an R-PDCCH to which the DM-RS for which themaximum transmission rank value of the RN-specific backhaul downlink isassumed (step S202). The RN decodes R-PDCCH control information byassuming DM-RS mapping for a case where the PDSCH region has the maximumtransmission rank value of the RN-specific backhaul downlink (stepS302). The eNB transmits the R-PDSCH to the RN (step S402). The RNdecodes the R-PDSCH (step S502). Optionally, as described above in ‘1.Method assuming maximum transmission rank of all backhaul link’, a2^(nd) R-PDCCH can be transmitted in the R-PDSCH region, and in thiscase, an actual transmission rank of the 2^(nd) R-PDCCH and a DM-RSoverhead depending thereon can be assumed to be a pre-defined value(e.g., a transmission rank 1 and a DM-RS overhead depending thereon).

The RN can know a transmission rank of an R-PDSCH when the RNsuccessfully decodes the R-PDCCH by using any one of the aforementionedmethods, i.e., ‘1. Method assuming maximum transmission rank of allbackhaul link’ and ‘2. Method assuming maximum transmission rank ofbackhaul downlink in RN-specific manner’. Therefore, regarding theR-PDSCH region, the eNB can map the DM-RS resource element according toan actual transmission rank of each RN. That is, regarding the R-PDSCHregion, the eNB maps a DM-RS and R-PDSCH data according to the actualtransmission rank instead of mapping them by assuming a maximumtransmission rank of a backhaul downlink or a maximum transmission rankof an individual backhaul downlink of each RN similarly to the case ofthe R-PDCCH region. Therefore, in the R-PDSCH region, the R-PDSCHresource element (i.e., a resource element to which data is mapped inthe R-PDSCH) can include a resource element which is not used in actualDM-RS transmission among DM-RS candidate resource elements. The RN candecode the R-PDCCH and thus can correctly decode the R-PDSCH accordingto the actual transmission rank.

Hereinafter, when it is said that the RN assumes the maximumtransmission rank of the backhaul link with respect to the R-PDCCHregion, it is the concept including the aforementioned methods, i.e.,‘1. Method assuming maximum transmission rank of all backhaul link’ and‘2. Method assuming maximum transmission rank of backhaul downlink inRN-specific manner’.

FIG. 17 shows an example of DM-RS resource elements assumed by an RN inan R-PDCCH region of a backhaul downlink subframe.

Referring to FIG. 17, the RN performs R-PDCCH decoding by assuming DM-RSresource elements arranged when an R-PDSCH is transmitted using amaximum transmission rank value. That is, for the DM-RS resourceelements in the R-PDCCH region, DM-RS resource elements arranged whenR-PDSCH transmission is performed with a rank 3 or a higher rank areassumed.

After decoding the R-PDCCH, the RN can know actual R-PDSCH transmissionrank value. Therefore, it is enough for the RN to decode the R-PDSCHregion by considering DM-RS resource elements depending on a rank valueof R-PDSCH transmission. In the example of FIG. 17, R-PDSCH transmissionis any one of rank-1 transmission or rank-2 transmission.

FIG. 18 shows an example of a DM-RS resource element of a backhauldownlink subframe.

Referring to FIG. 18, by assuming a DM-RS for a maximum transmissionrank value irrespective of an actual R-PDSCH transmission rank value, aneNB can allocate an R-PDCCH and an R-PDSCH to a resource element towhich the DM-RS is not allocated. By assuming a DM-RS for a maximumtransmission rank value of the R-PDSCH, an RN can perform R-PDCCH andR-PDSCH decoding on a resource element which does not overlap with aresource element to which the DM-RS can be allocated. That is, the eNBcan equally maintain a structure of the DM-RS in each slot of a backhauldownlink subframe. In this manner, the increase in complexity can beavoided, and implementation can be achieved more conveniently.

3. Precoding Matrix/Vector Applied to R-PDCCH and R-PDSCH

FIG. 19 shows an exemplary structure of a transmitter according to anembodiment of the present invention.

Referring to FIG. 19, the transmitter includes a MIMO processor 171, aprecoder 172, and a reference signal generator 173. The transmitter maybe a part of an eNB.

The MIMO processor 171 generates control information and data to betransmitted to an RN. The MIMO processor 171 generates R informationstreams (ISs) IS#1 to IS#R as the control information and data. Herein,R denotes the number of spatial layers.

The precoder 172 receives spatial streams (SSs) from the MIMO processor171 and generates transmit streams (TSs) TS#1 to TS#N_(T) by applying aprecoding matrix/vector. Herein, N_(T) is equal to the number of Txantennas.

The reference signal generator 173 generates a reference signalsequence, and provides the generated reference signal sequence to aninput or output of the precoder 172. A DRS used as the aforementionedDM-RS is provided to the input of the precoder 172, is subjected to theprecoder 172, and then is output by being included in the TS. That is,the DRS becomes a precoded reference signal. A CRS is added to theoutput of the precoder 172 and thus is included in the TS.

If the DRS is used for an R-PDCCH and an R-PDSCH, a precoding matrix forthe two channels (i.e., R-PDCCH and R-PDSCH) is required to support aprecoded reference signal. In this case, the eNB can configure aprecoding matrix/vector for the R-PDCCH as a subset of a precodingmatrix/vector used for the R-PDSCH.

For example, a precoding matrix W used in the R-PDSCH can be expressedby Equation 12 below.

$\begin{matrix}{W = {\left( {w_{0}\mspace{14mu} w_{1}\mspace{14mu}\ldots\mspace{14mu} w_{R\; - 1}} \right) = \begin{pmatrix}w_{01} & w_{11} & \ldots & w_{{({R - 1})}1} \\w_{02} & w_{12} & \ldots & w_{{({R - 1})}2} \\\vdots & \vdots & \ddots & \vdots \\w_{0{Nt}} & w_{1{Nt}} & \ldots & w_{{({R - 1})}{Nt}}\end{pmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$

Herein, w_(i) indicates an i^(th) column vector of a precoding matrix W(where i=0, . . . , R−1). If a rank of the R-PDSCH is 3, the precodingmatrix W can be expressed by (w₀, w₁, w₂). In this case, if a rank ofthe R-PDCCH is 1, a precoding vector for the R-PDCCH can be selected asany one of column vectors from a rank-3 precoding matrix of the R-PDSCH.That is, any one of w₀, w₁, and w₂ can be selected.

If a transmission rank of the R-PDCCH is given by X, the precodingvector can be selected by using various methods. For example, first Xcolumn vectors can be selected from a precoding matrix applied to theR-PDSCH, and last X column vectors can be selected from the precodingmatrix. Alternatively, any X column vectors can be selected from theprecoding matrix through explicit signaling.

The aforementioned method implies that a precoding vector/matrix used inthe R-PDCCH is a subset of a precoding matrix/vector used in theR-PDSCH. In addition, it implies that a Tx antenna port (or layer) of aDRS (i.e., DM-RS) is used in both the R-PDCCH and the R-PDSCH. That is,the R-PDCCH and the R-PDSCH are multiplexed such that resource elementsare exclusive in a time/frequency domain (which means that they areallocated to different resource elements), but it is not spatiallyexclusive.

Meanwhile, another method can be used by considering a fact that DM-RSresource elements are equally arranged to both-side slots at a slotboundary of a subframe and an R-PDCCH resource element to which theR-PDCCH is mapped exists only in a 1^(st) slot to avoid decoding delay.That is, a DM-RS resource element of the 1^(st) slot is used for R-PDCCHdemodulation, and a DM-RS resource element of a 2^(nd) slot is used forR-PDSCH demodulation. Then, a different precoding matrix can be appliedto the DM-RS according to a channel type, i.e., according to whether achannel is the R-PDCCH or the R-PDSCH. In order to support such amethod, an eNB can semi-statically signal an index of a DM-RS used inthe R-PDCCH through a high-layer signal, and can signal an index of aDM-RS used in the R-PDSCH in a corresponding R-PDCCH. If a DM-RSresource element of a different slot is used when demodulating theR-PDCCH and the R-PDSCH, there is no overlapping portion in the use oftwo DM-RSs even if the R-PDCCH and the R-PDSCH are demodulated by usingthe same DM-RS index. Therefore, a DM-RS resource element fordemodulating the R-PDCCH can be known irrespective of a rank of theR-PDSCH. An RN does not have to perform blind decoding to distinguish anR-PDCCH resource element and a DM-RS resource element.

Alternatively, the R-PDCCH can be transmitted by using one dedicatedDM-RS Tx antenna port which is not used by the R-PDSCH (when transmitdiversity is applied to the R-PDCCH, the R-PDCCH can be transmitted byusing two DM-RS Tx antenna ports). In this method, the R-PDCCH and theR-PDSCH are spatially multiplexed in an exclusive manner. In this case,the R-PDCCH can be transmitted by using a cyclic delay diversity (CDD)or transmit diversity scheme, for example, space-time block coding(STBC), space-frequency block coding (SFBC), or a combination of theSTBC and the SFBC.

Alternatively, the eNB can transmit a CRS in a subframe in which theR-PDCCH is transmitted. Further, the RN can demodulate the R-PDCCH byusing the CRS and can demodulate the R-PDSCH by using a DM-RS. Ingeneral, the CRS is transmitted across a full system band and across afull subframe. In an LTE-A subframe (e.g., an MBSFN subframe or a fakeMBSFN subframe), the eNB transmits the CRS only in a specific number offirst OFDM symbols. Herein, the MBSFN subframe or the fake MBSFNsubframe has the same structure as the MBSFN subframe for an multimediabroadcast and multicast service (MBMS), but is not a subframe used forthe MBMS. That is, the MBSFN subframe or the fake MBSFN subframe is asubframe for transmitting a backhaul signal by the eNB to the RN, and isa subframe for giving information indicating that it is a subframe notrequiring signal reception and measurement to a Ma-UE in a specificnumber of first OFDM symbols of a subframe and for transmitting abackhaul signal to the RN in subsequent OFDM symbols. In the LTE-Asubframe, the RN can assume that the CRS is located only in a resourceblock in which the R-PDCCH is transmitted (of course, the R-PDSCH canalso be included in the resource block), and then can demodulate theR-PDCCH. When the eNB reports the LTE-A subframe to the RN, the R-PDCCHcan be demodulated by using the CRS transmitted across the full systemband according to the aforementioned method. The R-PDCCH can betransmitted by the eNB by using a transmit diversity scheme such asSFBC, and the RN can perform demodulation by assuming that only the CRSexists in the resource block in which the R-PDCCH is transmitted. Then,the R-PDSCH can be demodulated by using the DM-RS. If a resource used inbackhaul transmission is spatially multiplexed with a resource used intransmission with respect to the Ma-UE (i.e., if multi-user MIMO is usedbetween the UE and the RN), the spatially multiplexed UE must receiveinformation indicating that a CRS exists in the subframe for multi-userMIMO transmission.

The eNB can transmit the R-PDCCH through the same antenna port as thatthrough which the CRS is transmitted. On the other hand, the R-PDSCH canbe transmitted through the same antenna port as that through which aDM-RS is transmitted. According to such a method, transmission can beperformed by using a CRS according to the transmit diversity or spatialmultiplexing scheme when the eNB transmits the R-PDCCH. At the sametime, the R-PDSCH can be precoded differently from the R-PDCCH or can besubjected to subband precoding.

Optionally, it may be difficult to puncture all resource elements bywhich a DM-RS can be transmitted in an R-PDCCH region. This is because areference signal overhead is excessively increased. In this case, only aDM-RS reference signal for a specific layer can be mapped in an OFDMsymbol duration in which the R-PDCCH is transmitted. Herein, thespecific layer may be a layer of up to a specific rank at which theR-PDCCH can be transmitted.

FIG. 20 shows an example in which an eNB maps a DM-RS resource elementto an R-PDCCH region and an R-PDSCH region according to a rank.

For example, if a transmission rank of the R-PDSCH is greater than orequal to 3 and a transmission rank of the R-PDCCH is limited to 2, onlya DM-RS (DRS) resource element for layers 1 and 2 is mapped in an OFDMsymbol duration in which the R-PDCCH is transmitted. On the other hand,in the R-PDSCH region, the DM-RS resource element for the layers 1 and 2and a DM-RS resource element for a layer 3 are both mapped. That is, aDM-RS of up to a transmission rank 2 is used in both the R-PDCCH regionand the R-PDSCH region, and a DM-RS of a rank 3 or a higher rank is usedonly in the R-PDSCH region. Resource elements for a CSI-RS can belocated in the same symbol as that in which a DM-RS is arranged becausethere is no dedicated symbol to be used by the CSI-RS. This is usefulfor an extended CP.

In order to prevent an R-PDCCH from being mapped to a resource elementto which a DM-RS can be mapped, the eNB can map the R-PDCCH to N OFDMsymbol durations in which the DM-RS is not included. According to thismethod, the R-PDCCH can be rapidly detected and decoded, and thus theR-PDSCH can also be rapidly detected and decoded. Herein, N can bedetermined by a high-layer signal. Alternatively, it can be apre-defined specific value.

If the CSI-RS can be mapped in an OFDM symbol duration in which theR-PDCCH is mapped, mapping of the R-PDCCH may vary depending on whetherthe CSI-RS is mapped. Therefore, the R-PDCCH may not be mapped in theOFDM symbol to which the CSI-RS is mapped. Alternatively, the R-PDCCHmay be mapped to another resource element other than a resource elementto which the CSI-RS is mapped. A second method is possible withoutadditional detection of a receiver and decoding complexity. This isbecause an RN can know whether the CSI-RS exists in the R-PDCCH regionby using system information.

The eNB transmits, to the RN, information regarding a type of a backhaulsubframe allocated to a backhaul link. The RN can perform demodulationby distinguishing a resource element to which the R-PDCCH is mappedaccording to the type of the backhaul subframe.

When the backhaul subframe in which the RN receives the R-PDCCH and theR-PDSCH is configured by the eNB as an MBSFN subframe or a fake MBSFNsubframe (hereinafter, an MBSFN subframe), the eNB does not perform CRStransmission in an OFDM symbol other than 1^(st) and 2^(nd) OFDM symbolsof the backhaul subframe. This implies that R-PDCCH resource elementmapping can vary depending on whether the eNB configures the backhaulsubframe as the MBSFN subframe. This is because a specific OFDM symbolduration to which a CRS resource element is inserted in the backhaulsubframe varies.

If the eNB signals to the RN a fact that a specific backhaul subframe isthe MBSFN subframe and thus the RN can know presence/absence of a CRS inadvance, the eNB can transmit the R-PDCCH by mapping it to a resourceelement other than a CRS resource element. More specifically, the eNBdoes not map the R-PDCCH to the CRS resource element in a subframe inwhich the CRS exists, whereas it is possible to map the R-PDCCH to aresource element to which the CRS can be arranged in a subframe in whichthe CRS does not exist (e.g., an MBSFN subframe).

If information on a type of a backhaul subframe is not given to the RN,the eNB maps the R-PDCCH to a resource element other than a resourceelement to which the CRS can be allocated irrespective of whether theCRS is actually transmitted. That is, if the RN cannot know in advancewhether the specific backhaul subframe is the MBSFN subframe, theR-PDCCH is transmitted by being mapped to the resource element otherthan the resource element to which the CRS can be allocated.

FIG. 21 shows an example of transmitting a plurality of R-PDCCHs todifferent spatial layers when the plurality of R-PDCCHs are multiplexedin one resource block in a frequency domain.

An R-PDSCH and an R-PDCCH can be multiplexed by being divided in afrequency domain. For example, this is a case where a resource elementof the R-PDCCH and a resource element of the R-PDSCH are not multiplexedin one resource block (i.e., 12 subcarriers) in the frequency domain,but are included in different resource blocks. Herein, the number ofresource elements included in one resource in the frequency domain maybe greater than the number of resource elements required to reliablytransmit the R-PDCCH by the eNB to the RN. In this case, the pluralityof R-PDCCHs transmitted to different RNs can be multiplexed in the sameresource block in the frequency domain. If the eNB uses a precoded DM-RSwhen transmitting the aforementioned plurality of R-PDCCHs, it may bedifficult to find a precoding vector for providing a good signal tointerference plus noise ratio (SINR) for a separated RN.

For this reason, the eNB can perform orthogonal spatial layertransmission between a plurality of RNs. For example, if two R-PDCCHs(i.e., R-PDCCH FOR RN#1, R-PDCCH FOR RN#2) are multiplexed to oneresource block in the frequency domain, each R-PDCCH can be transmittedin a different slot.

At the same time, each R-PDCCH can be transmitted in a different DM-RSantenna port. In practice, this has the same meaning as that differentR-PDCCHs are mapped to resource elements on different time/frequencydomains in one resource block. In order to apply different precoding toeach R-PDCCH, the eNB transmits each R-PDCCH through a different DM-RSantenna port. In this case, the R-PDCCH transmitted to each different RNis transmitted to a different spatial layer. Further, a DM-RS for eachR-PDCCH is transmitted in a resource element of the same time/frequencydomain and is multiplexed in a code region by the use of an orthogonalcode. According to this method, the number of resource elements of eachR-PDCCH can be prevented from being modified according to whether theplurality of R-PDCCHs are included in one resource block.

If the R-PDCCH and the R-PDSCH are transmitted in a physical resourceblock (PRB) pair, the number of transmission layers of the R-PDCCH andthe number of transmission layers of the R-PDSCHs may be different fromeach other (see FIG. 13). In this case, the eNB can perform transmissionby precoding some resource element groups for transmitting the R-PDCCHby the use of a precoding vector configured with a linear combination ofprecoding vectors of an R-PDSCH transmission layer, and can performtransmission by precoding the remaining resource element groups by theuse of a precoding vector configured with another linear combination ofthe precoding vectors of the R-PDSCH transmission layer.

For example, assume that the R-PDCCH has one transmission layer, and theR-PDSCH has K transmission layers. In this case, k layers (where k isany one of 0, 1, . . . , K−1) of the R-PDSCH are mapped to DM-RS antennaports n₀, n₁, . . . , n_(k-1). It is assumed herein that a precodingvector v_(m)=[v_(m,0) v_(m,1) . . . v_(m,P-1)] (where P is the number ofTx antenna ports) is commonly applied to a transmission layer m of theR-PDSCH and a DM-RS antenna port n_(m).

Then, resource elements used in R-PDCCH transmission can be grouped intoG resource element groups (i.e., R-PDCCH resource element groups).Resource element grouping is preferably performed in such a manner thatcontiguous resource elements in a time/frequency domain are not includedin the same group (herein, a grouping configuration can be predeterminedor can be signaled to the RN). A resource element group g (where g isany one natural number from 1 to G) has its combination weighta_(g)=[a_(g,0) a_(g,1) . . . a_(g,k-1)]. The combination weight can bepredetermined or can be signaled to the RN.

When the eNB transmits the R-PDCCH, a signal mapped to resource elementof the resource element group g is precoded by the use of a precodingvector a_(g,0)*v₀+a_(g,1)*v₁+ . . . +a_(g,k-1)*v_(k-1). That is,precoding is performed by the use of a linear combination vector inwhich a combination weight of the resource element group g is applied toprecoding vectors of the R-PDSCH. In other words, the R-PDCCH resourceelement groups are precoded by linear combination vectors in which theircombination weights are applied to the R-PDSCH precoding vectors.According to this method, a greater spatial diversity gain can beobtained when the eNB transmits the R-PDCCH.

In the aforementioned example, the RN can demodulate the R-PDCCH byusing the following procedure.

1. An effective channel (i.e., a channel multiplied by a precodingvector) of each R-PDSCH transmission layer is estimated.

2. An effective channel of each R-PDCCH resource element group is foundby applying a combination weight of each R-PDCCH resource element group.

3. R-PDCCH resource elements are demodulated from an effective channelof a corresponding R-PDCCH resource element group.

A combination weight for all R-PDCCH resource elements may be, forexample, [1 0 . . . 0]. This implies that a precoding vector of anR-PDSCH transmission layer 0 (i.e., a DM-RS antenna port of an R-PDSCHtransmission layer 0) is used for the R-PDCCH.

For another example, g=k, a₀=[1 0 . . . 0], a₁=[0 1 0 . . . 0], . . . ,a_(g)=[0 . . . 0 1]. In this case, a precoding vector (and a DM-RSantenna port) of an R-PDSCH transmission layer g is used for the R-PDCCHresource element group g, which implies that a precoding vector of eachR-PDSCH transmission layer and a DM-RS antenna port are applied to theR-PDCCH. Alternatively, if g is predetermined or is a specific valuesignaled to the RN, it is possible to use combination weights a₀=[1 0 .. . 0], a₁=[0 1 0 . . . 0], . . . , a_(g)=[0 . . . 0 1].

For another example, as a combination weight of each resource elementgroup (i.e., R-PDCCH resource group), a circular shift of a specificcommon vector is used. For example, a discrete Fourier transform (DFT)sequence a_(g)=[exp(0*j2πg/k) exp(1*j2π*g/k) . . . exp((k−1)*j2πg/k)]can be used as a combination weight of a resource element group. If thenumber of R-PDSCH transmission layers is 2 and the number of R-PDCCHresource element groups is 2, a₀=[1 1] and a₁=[1−1] can be used. Thisimplies that (v₀+v₁) is applied to a resource element group 0, and(v₀−v₁) is applied to a resource element group 1. Alternatively, a DFTsequence a_(g)=[exp(0*j2πg/L) exp(1*j2π*g/L) . . . exp((L−1)*j2πg/L)]can be used as a combination weight of a resource element group. Herein,L can be a predetermined value or a value signaled to the RN.

The aforementioned method, i.e., the method of using a combination ofone or more R-PDSCH DM-RS sequences as the R-PDCCH DM-RS sequence, isalso applicable to a case where a plurality of R-PDCCHs (or some of aplurality of R-PDCCHs) are transmitted in one PRB pair.

For example, it is assumed that L different R-PDCCHs are transmitted inone PRB pair (herein, L may be a predetermined value or a value signaledto the RN). Further, it is also assumed that k DM-RS antenna ports areused for the L R-PDCCHs (herein, k may be a predetermined value or avalue signaled to the RN). Then, signals transmitted from differentR-PDCCHs are mapped to different resource elements. That is, the signalsare mapped to orthogonal time/frequency resources. Resource elementsused in R-PDCCH transmission are grouped similarly to the aforementionedmethod. The resource element group g can have a combination weighta_(g), and an R-PDCCH signal transmitted in the resource element group gcan be precoded by a precoding vector a_(g,0)*v₀+a_(g,1)*v₁+ . . .+a_(g,k-1)*v_(k-1).

For example, assume that L=2, k=2, a₀=[1 1], a₁=[1−1]. Further, it isalso assumed that among resource elements of the PRB pair, even resourceelements (e.g., resource elements 0, 2, 4, . . . ) are included in aresource element group 0, and odd resource elements (i.e., resourceelements 1, 3, 5, . . . ) are included in a resource element group 1.Then, two R-PDCCHs can be transmitted as follows.

1. The resource element 0 is used in the R-PDCCH 0, and a precodingvector (v₀+v₁) can be used. 2. The resource element 1 is used in theR-PDCCH 0, and a precoding vector (v₀−v_(i)) can be used. 3. Theresource element 2 is used in the R-PDCCH 1, and a precoding vector(v₀+v₁) can be used. 4. The resource element 3 is used in the R-PDCCH 1,and a precoding vector (v₀−v₁) can be used. The aforementioned resourceelement allocations 1 to 4 are repeated for all resource elements of thePRB pair.

When an R-PDCCH is demodulated by using a DM-RS to support effectivemulti-user MIMO in a backhaul resource, the eNB can indicate an antennaport of a DM-RS of the R-PDCCH for each RN. Alternatively, the eNB canindicate a scramble ID of a DM-RS antenna port 0 of the R-PDCCHtransmitted to each RN. Alternatively, the eNB can indicate acombination of the scramble ID and the DM-RS antenna port of the R-PDCCHtransmitted to each RN. The scramble ID of the DM-RS antenna port is inregard to a DM-RS antenna port different from a DM-RS antenna port usedto schedule a different multi-user MIMO resource in a space domain. Theaforementioned DM-RS index can be given by the DM-RS antenna port, thescramble ID, or a combination of the two.

R-PDCCH transmission to the RN can be performed by the eNB by using aDM-RS antenna port which is not predetermined. This implies that the RNblindly detects the R-PDCCH by using a DM-RS antenna port (and/or ascramble ID) which cannot be pre-known in the R-PDCCH resource.According to this method, instead of transmitting DM-RS antenna port(and/or scramble ID) information of the R-PDCCH and the R-PDSCH to theRN in advance, the eNB may dynamically perform multi-user MIMOtransmission with respect to resources of the RN.

When the RN performs blind detection on the R-PDCCH, it may be effectiveto restrict a DM-RS antenna port used for R-PDCCH transmission. Forexample, in order to demodulate the R-PDCCH, it can be restricted suchthat only a DM-RS antenna port 0 and a DM-RS antenna port 1 are used.According to this example, a reference signal overhead can be minimizedby allowing two antenna ports to share the same resource elements and tobe divided along a code axis (i.e., CDM).

Alternatively, it can be restricted such that only the DM-RS antennaports 0 and 2 are used for R-PDCCH demodulation. According to thismethod, there is an advantage in that an R-PDSCH transmission rank foreach RN can be extended up to 2 in multi-user MIMO. The RN candemodulate its R-PDCCH by using the DM-RS antenna port 0 and at the sametime, can demodulate an R-PDSCH received with a transmission rank 2 byusing the DM-RS antenna ports 0 and 2. The RN which modulates itsR-PDCCH by using the DM-RS antenna port 2 can demodulate an R-PDSCHreceived with a transmission rank 2 by using the DM-RS antenna ports 2and 3. For this operation, the RN performs demodulation by assuming thatan R-PDCCH signal is mapped with a maximum DM-RS overhead (e.g., byassuming that 24 resource elements are mapped in a resource block).However, if the entire transmission rank is less than or equal to 2, anactual DM-RS overhead may be further decreased (in a case of mapping to12 resource elements in a resource block). As a result, a 1^(st) slot inwhich the R-PDCCH is transmitted may have a higher DM-RS overhead than a2^(nd) slot in which the R-PDSCH is transmitted.

FIG. 22 is a block diagram showing an eNB and an RN.

An eNB 100 includes a processor 110, a memory 120, and a radio frequency(RF) unit 130. The processor 110 implements the proposed functions,procedures, and/or methods. That is, the processor 110 transmitsinformation regarding a DM RS used for demodulation of an R-PDCCHthrough a high-layer signal, and transmits information regarding adedicated reference signal for an R-PDSCH in the R-PDCCH. The processor110 transmits a maximum transmission rank value of a backhaul downlinkor a maximum transmission rank value of an RN-specific backhaul downlinkthrough a high-layer signal, and transmits to the RN an R-PDCCH forwhich the rank value is assumed. The memory 120 coupled to the processor110 stores a variety of information for driving the processor 110. TheRF unit 130 coupled to the processor 110 transmits and/or receives aradio signal.

An RN 200 includes a processor 210, a memory 220, and an RF unit 230.The processor 210 receives a maximum transmission rank value of abackhaul downlink or a maximum transmission rank value of an RN-specificbackhaul downlink through a high-layer signal such as an RRC message,receives control information from the eNB through a control region, anddecodes the control information. In the process of decoding the controlinformation, the processor 210 decodes an R-PDCCH by assuming a maximumtransmission rank value of a backhaul downlink or a maximum transmissionrank value of an RN-specific backhaul downlink. After decoding theR-PDCCH, the R-PDSCH can be decoded by using decoded controlinformation. Layers of a radio interface protocol can be implemented bythe processor 210. The memory 220 coupled to the processor 210 stores avariety of information for driving the processor 210. The RF unit 230coupled to the processor 210 transmits and/or receives a radio signal.

The processors 110 and 210 may include an application-specificintegrated circuit (ASIC), a separate chipset, a logic circuit, a dataprocessing unit, and/or a converter for mutually converting a basebandsignal and a radio signal. The memories 120 and 220 may include aread-only memory (ROM), a random access memory (RAM), a flash memory, amemory card, a storage medium, and/or other equivalent storage devices.The RF units 130 and 230 may include one or more antennas fortransmitting and/or receiving a radio signal. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememories 120 and 220 and may be performed by the processors 110 and 210.The memories 120 and 220 may be located inside or outside the processors110 and 210, and may be coupled to the processors 110 and 210 by usingvarious well-known means.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method of decoding a backhaul downlink signal,performed by a relay node (RN), the method comprising: receiving ahigher layer signal indicating a maximum transmission rank from a basestation (BS); receiving control information containing a resourceallocation for downlink data through a relay control channel from theBS; demodulating the control information; and receiving the downlinkdata through a data channel based on the control information, whereinthe control information is mapped to resource elements (REs) which donot overlap with user equipment-specific reference signal (URS) REs in acontrol region which is used for the relay control channel transmissionof the BS, the URS REs being reserved REs for URSs according to themaximum transmission rank.
 2. The method of claim 1, wherein the controlregion is located in a first slot of a subframe, the subframe comprisestwo slots and each slot of the two slots comprises a plurality of REs.3. The method of claim 1, wherein each of the plurality of REs comprisesan orthogonal frequency division multiplexing (OFDM) symbol in timedomain and a subcarrier in frequency domain.
 4. The method of claim 1,wherein the maximum transmission rank is equal to a maximum rank thatcan be transmitted between the BS and the RN.
 5. The method of claim 1,wherein the data channel is received in a same subframe in which therelay control channel is received.
 6. The method of claim 5, wherein therelay control channel is received in a first slot of the same subframeand the data channel is received in a second slot of the same subframe.7. The method of claim 1, wherein the higher layer signal is a radioresource control (RRC) message.
 8. A relay node (RN) comprising: a radiofrequency (RF) unit configured to transmit and receive a radio signal;and a processor coupled to the RF unit, wherein the processor isconfigured to: receive a higher layer signal indicating a maximumtransmission rank from a base station (BS); receive control informationcontaining a resource allocation for downlink data through a relaycontrol channel from the BS; demodulate the control information; andreceive the downlink data through a data channel based on the controlinformation, wherein the control information is mapped to resourceelements (REs) which do not overlap with user equipment-specificreference signal (URS) REs in a control region which is used for therelay control channel transmission of the BS, the URS REs being reservedREs for URSs according to the maximum transmission rank.
 9. The RN ofclaim 8, wherein the control region is located in a first slot of asubframe, the subframe comprises two slots and each slot of the twoslots comprises a plurality of REs.
 10. The RN of claim 8, wherein eachof the plurality of REs comprises an orthogonal frequency divisionmultiplexing (OFDM) symbol in time domain and a subcarrier in frequencydomain.
 11. The RN of claim 8, wherein the maximum transmission rank isequal to a maximum rank that can be transmitted between the BS and theRN.
 12. The RN of claim 8, wherein the data channel is received in asame subframe in which the relay control channel is received.
 13. The RNof claim 12, wherein the relay control channel is received in a firstslot of the same subframe and the data channel is received in a secondslot of the same subframe.
 14. The RN of claim 8, wherein the higherlayer signal is a radio resource control (RRC) message.